In the past, there were three systems used for controlling the calibration of frequency response of AC amplifiers in precision instruments.
The first method used low cost commercial components which included variable capacitors, resistors, and/or inductors. These instruments were generally calibrated only in the factory by skilled technicians very carefully adjusting component values to the required degree of precision. In a typical high precision instrument, there are at least as many high frequency calibration points as there are ranges. Thus, a tremendous number of adjustments were required which meant that a precision instrument was and still is very expensive in terms of the skilled manpower required to produce it.
Further, mechanical disturbances in the factory from putting on and taking off the instrument's protective covers would often result in shifts of the precision settings. In the field, unless extreme care were taken to avoid subjecting the precision instrument to vibration or shock, the settings would also shift and require the instruments to be recalibrated.
The second method, which eliminated the need for manual adjustments, was to use extremely high precision components in matched sets or selectively choosing parts having the precise values required. Unfortunately, the matching or selecting process resulted in extremely expensive parts. Further, if one of a matched set of components failed, the entire matched set would have to be replaced and the instrument again recalibrated to the desired frequency response.
The third method, which is used by a few highly skilled companies in the industry, is to build the circuitry as a hybrid or thin film circuit and using an active laser or abrasive trimming of the circuit to obtain the necessary values. This method naturally requires a high degree of technological sophistication to produce the basic circuits as well as the expenditure of capital to purchase the expensive laser or precision abrasive trimming equipment.
Heretofore, it has not been possible to perform all calibration of frequency response using computer controlled automatic test equipment. This equipment would require no skilled labor or special training to operate, and could allow calibration to be performed relatively quickly. There has been no method by which in sitsu calibration could be performed so all previous methods required calibration prior to putting on the final shields, covers or guards on the instrument.
Further, no method has been developed which would allow the use of low cost commercial components without requiring expensive manual adjustment or machining equipment.